The Congressional Black Caucus PAC endorsed Hillary Clinton for president Thursday, saying Mrs. Clinton had done the most to support blacks seeking office and the Democratic Party.

“The partner that the CBC PAC has had over the years to elect Democrats has been Hillary Clinton,” said Rep. Gregory Meeks, New York Democrat and chairman of the caucus’s political action committee, in announcing the endorsement. “When we needed someone to come to rally Democrats and especially African-Americans at the request of the CBC PAC Hillary Clinton has been there.”

Mrs. Clinton won the endorsement of at least 36 members of the caucus — or 90 percent — with no members voting for Sen. Bernard Sanders and some abstaining.

Rep. Keith Ellison, a member of the caucus from Minnesota, took to Twitter Thursday to clarify that the PAC was the entity giving the endorsement, not the CBC itself.

“Cong’l Black Caucus (CBC) has NOT endorsed in presidential. Separate CBCPAC endorsed withOUT input from CBC membership, including me,” he tweeted.

Rep. James E. Clyburn of South Carolina, the No. 3 House Democratic leader and most prominent Democrat in the state, also refused to give an endorsement to Mrs. Clinton, at least for now.

“I will follow their lead and make a decision sometime soon since they are endorsing today. I did not want to get out in front of them,” Mr. Clyburn said of the caucus PAC’s decision.

Mr. Clyburn remained neutral in the 2008 primary contest and intervened when Bill Clinton made what some viewed as disparaging comments at the time against President Obama. Mr. Clyburn didn’t attend Thursday’s press conference endorsing Mrs. Clinton.

Rep. Elijah Cummings, the ranking member of the House Oversight and Government Reform Committee, abstained from the CBC PAC vote, citing his role in the Benghazi hearings, Buzzfeed News reported.

The endorsement comes as a boon to Mrs. Clinton as she scrambles to solidify the black vote in South Carolina, a must-win state for her campaign after virtually tying Mr. Sanders in Iowa and losing by double-digits to the senator in New Hampshire.

The CBC will send its surrogates into the field to help rally voters for Mrs. Clinton before South Carolina’s Democratic primary on Feb. 27.

Mrs. Clinton said she was “proud to stand shoulder-to-shoulder” with her “close friends” at the CBC in fighting for the progress of blacks.

“I’m honored to have earned the endorsement of the Congressional Black Caucus PAC, which has been fighting for enduring progress for almost 50 years,” Mrs. Clinton said in a statement Thursday. “The CBC PAC knows we need to elect a president who can take on all parts of the job and build on the progress we’ve made under President Obama — not let it get ripped away.”

“I would suggest to those of you who have not yet made up your minds, or maybe even some of you who have made up your minds, to maybe consider and reconsider what it is that Bernie Sanders offers,” Mr. Belafonte said in a video endorsing Mr. Sanders. “He offers us a chance to declare unequivocally that there is a group of citizens who have a deep caring for where are nation goes and what it does in the process of going.”

Mr. Sanders had breakfast Wednesday with the Rev. Al Sharpton in New York City, meeting at the same restaurant where Mr. Sharpton sat down with then-Sen. Barack Obama in 2008.

While Mr. Sharpton hasn’t offered a formal endorsement of Mr. Sanders, the meeting is a sign that black leaders could be on the verge of lining up behind the Vermont senator. Former NAACP President Benjamin Jealous already has endorsed Mr. Sanders, saying the senator has been a consistent, clear voice against racism.